In recent years, researches have been carried out in order to introduce electric vehicles, hybrid vehicles and fuel cell vehicles due to the rising price of petroleum resources and rising global earth environment protection movement. Batteries as an auxiliary power are essential for these driving systems. In addition, high output batteries capable of responding to the sudden starting and sudden acceleration of vehicles have been desired. Further, batteries having high energy density are desired from the viewpoints of weight load to the vehicles and enhancement in fuel consumption. Because of these factors, lithium ion secondary batteries having highest energy density and capable of expressing high output have been highly expected among secondary batteries.
Generally, a lithium ion secondary battery has electrodes composed of a positive electrode containing a lithium ion-containing oxide and a negative electrode containing a carbon material. Since the conductivity of the lithium ion-containing oxide (positive electrode active material) itself is low in the positive electrode, conductive materials such as carbon black, a carbon fiber and the like are added in order to enhance the conductivity. In recent years, VGCF (registered trademark) having a fiber diameter of nanometer order, which is obtained by a gas phase process, and a carbon nanotube have been developed for the carbon fiber. The application of the carbon fiber to batteries has been considered.
For example, Patent Document 1 discloses an example in which a positive electrode active material, a carbon nanotube, graphite and a binder are mixed to form a paste, and the paste is applied to produce a positive electrode.
Patent Document 2 discloses a technique for mixing a spherical positive electrode active material having a particle diameter of 5 to 30 μm and a carbon nanofiber while mechanically applying a strong shearing force thereto by a centrifugal ball mill, dividing the carbon nanofiber and making the divided carbon nanofiber adhere to the surface of the active material.
Furthermore, Patent Document 3 discloses a process for producing a composite material for positive electrodes of lithium batteries in which a positive electrode active material obtained by mixing and calcining a transition metal compound and a lithium compound, and a conductive auxiliary agent such as carbon powder are spray-dried to be formed into a composite material.    Patent Document 1: Japanese Patent Application Laid-Open No. 11-283629    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-164859    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-173777